Vehicles employing a turbocharged or supercharged engine and pneumatic braking systems are well known. The pneumatic braking system typically has an air compressor, a governor for controlling the operating cycle of the air compressor, an air dryer and a reservoir for holding pressurized air for delivery to the brakes. The air compressor operates successively in a loading (compressing) mode and an unloading (non-compressing) mode and typically obtains supercharged air, typically about 25 psig, from the air intake manifold of the engine.
When the pressure in the air reservoir falls below a predetermined minimum pressure, typically about 100 psig, the governor causes the air compressor to operate in the loading mode to compress air for storage in the air reservoir. Before storing the air in the air reservoir, the compressed air from the air compressor passes through an air dryer which removes moisture and other contaminants therefrom. When the air reservoir reaches the desired pressure, typically about 120 psig, the governor causes the air compressor to operate in the unloading mode.
In the unloading mode of many typical compressor systems, the supercharged air from the engine passes through the air compressor and into the air dryer and is then exhausted into the atmosphere by the air dryer purge valve. The unrestricted flow of supercharged air from the engine manifold is wasteful and results in a loss of engine power and efficiency, especially in turbocharged engines which require the supercharged air for engine power.
Attempts to prevent the loss of supercharged air or turbocharged boost during the unloading cycle have been largely unsuccessful. Attempts to utilize an external isolation valve to close the intake port of the air dryer have proven to be relatively unsuccessful, in part because they are relatively expensive to manufacture and install. Attempts to utilize a single internal isolation valve which acts as a purge valve and a turbosaver valve have not been entirely successful because the metal to metal contact between the metal turbosaver valve and the piston seat typically results in leakage problems. In addition, since both the purge valve and the turbosaver valve are interconnected, failure of one of the valves results in failure of the other valve.
It will also be appreciated that many conventional dryers typically have internal chambers which are attached together using a plurality of retaining bolts to store compressed gas during the loading and unloading modes. Since the compressed gas typically has relatively high pressures, the internal pressures exerted on the internal compartments of the dryer may be quite substantial. When properly released, for example, during the unloading mode, the compressed gas is harmless. However, premature dismantling of the gas dryer may cause the compressed gas to be released in an uncontrolled and undesirable manner. Tampering with the dryer or improper removal of the retaining bolts may result in an immense release of compressed gas, causing sections of the dryer to be propelled with considerable force. In some circumstances, untrained personnel dismantle the dryer when the dryer malfunctions because the retaining bolts are accessible and easily disassembled with readily available tools. The problems are enhanced when the untrained personnel improperly install the retaining bolts or neglect to reassemble all of the retaining bolts, not realizing the importance of proper assembly and maintenance. These problems are recognized by the industry as evidenced by some gas dryers which may have safety release valves or instructions attached to the dryer informing those inspecting or repairing the air dryer of the presence of the considerable internal pressures and the consequences resulting from the improper removal or tampering with the retaining bolts.